1. Field of the Invention
The present invention relates to a reflection-type exposure mask which is used for EUV (Extreme Ultra-Violet) exposure, and a method of manufacturing a semiconductor device.
2. Description of the Related Art
In recent years, for further microfabrication of devices, the development of EUV exposure techniques has been promoted. In EUV exposure, a reflection-type exposure mask is used. In general, the reflection-type exposure mask comprises a multilayer reflection film which reflects EUV light, and an absorber pattern which is provided on the multilayer reflection film and absorbs EUV light. This absorber pattern has a pattern corresponding to a pattern that is to be formed on a wafer.
EUV light enters obliquely onto the reflection-type exposure mask. Thus, if the thickness of the absorber pattern is great (i.e. the aspect ratio is high), a phenomenon called “shadowing effect” occurs. The shadowing effect affects the dimensions of a transfer pattern. In order to reduce a dimensional error of the transfer pattern due to the shadowing effect, it is necessary to reduce the thickness of the absorber pattern. As a method therefor, there is known a method using a halftone-type phase shift mask (Proc. of SPIE2008 T. Kamo, “Effects of mask absorber thickness on printability in EUV Lithography with high resolution resist”).
The following items (1) and (2) have been understood by a reflectance simulation using optical constants (e.g. refractive index n and attenuation coefficient k) of various materials as parameters, and by a lithography simulation using an exposure condition and a mask condition as parameters.
(1) The case of realizing a halftone-type phase shift mask by using Ta-based material or Cr-based material as the material of the absorber pattern.
when the phase difference between an EUV light reflected by the multilayer reflection film (multilayer film reflection light) and an EUV light not absorbed by the absorber pattern and reflected by the absorber pattern (absorber reflection light) is 180°, the reflectance ratio therebetween (the reflectance of absorber reflection light/the reflectance of multilayer reflection light) needs to be set in a range of 1% to 3%.
(2) The case of realizing a halftone-type phase shift mask by using CrN-based material as the material of a buffer layer and using TaBN-based material as the material of the absorber pattern.
The total thickness of the buffer layer and the absorber pattern is reduced to 61 nm from a conventional standard of 80 nm. In this manner, by reducing the thickness of the absorber pattern, etc. so as to realize the halftone-type phase shift mask, the shadowing effect can be reduced with respect to the pattern dimensions of about 32 nm HP to 22 nm HP. Thereby, pattering can be performed without degrading a transfer image.
However, if the target dimension reaches a finer dimensional region, the tendency becomes obvious that a line pattern extending perpendicular to the incident direction of EUV light has a lower contrast value of an optical image, which is formed on a wafer, than a line pattern extending in parallel to the incident direction of EUV light. That is, if the target dimension reaches a finer dimensional region, there arises such a problem that it is difficult to form an optical image having an adequate contrast value on the wafer by using a conventional reflection-type exposure mask.